request_channels does not work for mpeg program stream

[Carl Eugen Hoyos]

-request_channels works fine for many tested containers, does not work for mpeg program stream

$ ./ffmpeg -i fate-suite/svq3/Vertical400kbit.sorenson3.mov -ac 6 -qscale 2 -acodec ac3 out.mpg

$ ./ffmpeg -request_channels 2 -i out.mpg
ffmpeg version N-42547-g7543fd8 Copyright (c) 2000-2012 the FFmpeg developers
  built on Jul 16 2012 07:44:51 with gcc 4.3.2
  configuration: --cc=/usr/local/gcc-4.3.2/bin/gcc --enable-gpl --enable-libopenjpeg --enable-libvorbis --enable-libspeex --enable-libmp3lame --enable-libtheora --extra-ldflags=-lm --enable-libvpx --enable-libxavs --enable-x11grab
  libavutil      51. 65.100 / 51. 65.100
  libavcodec     54. 40.100 / 54. 40.100
  libavformat    54. 16.104 / 54. 16.104
  libavdevice    54.  1.100 / 54.  1.100
  libavfilter     3.  2.100 /  3.  2.100
  libswscale      2.  1.100 /  2.  1.100
  libswresample   0. 15.100 /  0. 15.100
  libpostproc    52.  0.100 / 52.  0.100
[mpeg @ 0x904d4a0] max_analyze_duration 5000000 reached at 5000000
Input #0, mpeg, from 'out.mpg':
  Duration: 00:00:43.57, start: 0.994200, bitrate: 1507 kb/s
    Stream #0:0[0x1e0]: Video: mpeg1video, yuv420p, 320x240 [SAR 1:1 DAR 4:3], 104857 kb/s, 30 fps, 30 tbr, 90k tbn, 30 tbc
    Stream #0:1[0x80]: Audio: ac3, 44100 Hz, 5.1(side), s16, 448 kb/s
At least one output file must be specified

I also tested avi, mkv, nut and mpegts and all report stereo / downmixing works for all other containers I tested